As is well known, the torque generated by an internal combustion engine, such as a 4-stroke engine has a maximum at a certain rotational speed of the engine and drops at increasing as well as decreasing numbers of revolution. The reduced torque in the range of low rotational speeds is disadvantageous especially with motor vehicles because the acceleration is wanting.
Usually a change-speed gear including several transmission stages is provided to displace the engine operation predominantly into a rotational speed range in which the engine still provides a relatively high torque. This useful measure which avoids overdimensioned piston displacement and the correspondingly high fuel consumption, however, solves the problem in part only because the number of transmission stages must be limited in consideration alone of the frequency of shifting. Consequently the problem remains of maintaining the most uniform torque possible within a wider range of rotational speeds.
A known means for influencing the torque of an internal combustion engine, especially also the course of the torque as plotted over the rotational speed of the engine, resides in supercharging the engine. In this case the boost takes care that an increased volume is supplied to the combustion chambers as compared to the suction mode, and this leads to increased performance without any increase in piston displacement. As is well known, this may be influenced also by cam control and thus the duration of the supply phase and its position with respect to the ignition time.
It is known to provide superchargers driven by the engine shaft, and these may either be of the flow type (centrifugal compressors) or of the displacement type (vane cell compressor, Roots blower). Although these mechanical superchargers driven by the engine shaft take up part of the engine output, they have the advantage over the known exhaust gas turbochargers of being able to become effective substantially without any delay in response.
Superchargers usually are driven at a much higher speed of rotation than the engine speed and this makes it necessary to provide a step-up gear between the engine shaft and the supercharger. It is also known with mechanical superchargers of the flow type to employ a gear of variable transmission ratio controlled in response to the rotational speed, for instance by means of a centrifugal governor. In this manner sufficient boost for the desired torque increase may be made available, especially at lower rotational speeds of the engine without requiring any wasteful discharge of boost at higher engine speeds. A corresponding known embodiment, therefore, may be regarded as being suitable in the first place to provide a convenient engine characteristic, especially in respect of the course of the torque in consideration of the rotational speed of the engine without any increase in piston displacement and at low fuel consumption.
It is known to connect a mechanically driven supercharger to the engine shaft by a magnetic clutch so that the supercharger may be switched on in addition rather than operating permanently together with the engine. Even in case of constant running together with the engine the supercharger may be so designed or the boost so controlled by discharge that the boosting effect substantially occurs at those engine speeds at which it is required. However this has the effect of deteriorating the efficiency or increasing the fuel consumption so that switching on of the supercharger, as required, appears to be advantageous.
With the known design including a magnetic clutch between the engine and the supercharger, the supercharger is switched on by way of the accelerator pedal after the latter has been pressed down during the suction mode of the engine until the throttle flap has become fully open. This means that the suction mode is continued up to full power and then an additional supercharging torque is made available by kick-down. However, this supercharging torque cannot be metered or applied differentiated in accordance with the respective rotational speed of the engine. This full switch of additional engine performance caused by the supercharging is not entirely without difficulty. Especially the missing possibility of metering has the effect that a motor vehicle travelling, for instance at full power in the suction mode at a velocity of 180 km/h is accelerated by the torque applied additionally to a velocity of, for example, 220 km/h because it is only at this point that the additional torque is used up by the additional road resistance caused by the velocity. Therefore, it is not possible to travel at a constant velocity of, for instance, 200 km/h.